1. Field of the invention
The present invention relates to a drive state control apparatus for a vehicle which can perform changeover between a two-wheel drive state and a four-wheel drive state.
2. Description of the related art
Conventionally, there has been widely known a transfer which includes an input shaft, a first output shaft, and a second output shaft (see, for example, Japanese Patent No, 3650255). Such a transfer includes a first changeover mechanism which can be switchable between a “first state” (corresponding to a two-wheel drive state) in which a power transmission system is formed only between the input shaft and the first output shaft, and a “second state” (corresponding to a four-wheel drive state) in which a power transmission system is formed between the input shaft and the first output shaft, and another power transmission system is formed between the input shaft and the second output shaft. This first changeover mechanism may be a multi-disc clutch mechanism, a dog-type (a spline-engagement type) changeover mechanism, or the like.
The input shaft is connected to an output shaft of a transmission connected to an engine of a vehicle. In general, the first and second output shafts are respectively connected to rear-wheel-side and front-wheel-side propeller shafts of the vehicle. The front-wheel-side propeller shaft is connected to left and right front wheels via a front-wheel-side differential, and the rear-wheel-side propeller shaft is connected to left and right rear wheels via a rear-wheel-side differential.
In the case of the vehicle equipped with such a transfer, when the first changeover mechanism is in the “first state”, there is established a “two-wheel drive state” in which a power transmission system is formed only between the engine and the left and right rear wheels. Meanwhile, when the first changeover mechanism is in the “second state”, there is established a “four-wheel drive state” in which a power transmission system is formed between the engine and the left and right rear wheels and another power transmission system is formed between the engine and the left and right front wheels. In this way, the “two-wheel drive state” and the “four-wheel drive state” are selectively established in accordance with the state of the first changeover mechanism.
Incidentally, there has been also widely known a vehicle which is equipped with the above-described transfer and a second changeover mechanism which is interposed in the axle of one of the left and right front wheels (a specific wheel). The second changeover mechanism is configured to be selectively switched to a “connected state” in which a power transmission system is formed between the specific wheel and the front-wheel-side differential, or a “disconnected state” in which no power transmission system is formed between the specific wheel and the front-wheel-side differential. The second changeover mechanism may be a dog-type (a spline-engagement type) changeover mechanism or the like.
In the case of a vehicle which includes not only the above-described transfer but also the above-described second changeover mechanism, when the vehicle is in the “four-wheel drive state” the first changeover mechanism is in the “second state,” and the second changeover mechanism is in the “connected state.” Meanwhile, when the vehicle is in the “two-wheel drive state,” the first changeover mechanism is in the “first state,” and the second changeover mechanism is in the “disconnected state.” As a result, when the vehicle is traveling in the “two-wheel drive state,” idle rotation of the front-wheel-side propeller shaft can be prevented (restrained).
Accordingly, in the “two-wheel drive state,” drive energy required for idle rotation of the front-wheel-side propeller shaft having a relatively large moment of inertia becomes unnecessary. As a result, fuel efficiency can be improved as compared with a vehicle which is not equipped with such a second changeover mechanism (that is, a vehicle in which idle rotation of the front-wheel-side propeller shaft occurs in the “two-wheel drive state”).
In the following description, there is assumed a case where a changeover condition for changeover from the “two-wheel drive state” to the “four-wheel drive state” is satisfied when a vehicle which includes the above-described second changeover mechanism in addition to the above-described transfer is traveling in the “two-wheel drive state.” In this case, an operation of switching the first changeover mechanism from the “first state” to the “second state” (hereinafter referred to as “changeover operation”) is performed, and an operation of switching the second changeover mechanism from the “disconnected state” to the “connected state” (hereinafter referred to as “connecting operation”) is performed. In the following description, in order to facilitate explanation, a portion of the axle of the specific wheel located between the second changeover mechanism and the specific wheel will be referred to as the “first axle”; and a portion of the axle of the specific wheel located between the second changeover mechanism and the front-wheel-side differential will be referred to as the “second axle.”
In particular, in the case where the second changeover mechanism is constituted by a dog-type (a spline-engagement type) changeover mechanism, in order to smoothly perform the connecting operation of the second changeover mechanism without use of a rotation synchronizing apparatus (synchronizer), the rotational speeds of the first and second axles must be (approximately) equal to each other during the connecting operation. In order that the rotational speeds of the first and second axles become (approximately) equal to each other, the rotational speed of the front-wheel-side propeller shaft must be (approximately) equal to a “value obtained by multiplying the rotational speed of the left and right front wheels by a differential gear ratio.”
As described above, in the “two-wheel drive state,” the front-wheel-side propeller shaft is (substantially) stopped. Accordingly, before completion of the changeover operation of the first changeover mechanism, there may arise a situation where the rotational speed of the front-wheel-side propeller shaft is smaller than the rotational speed of the rear-wheel-side propeller shaft, and the rotational speed of the front-wheel-side propeller shaft is smaller than the value obtained by multiplying the rotational speed of the left and right front wheels by the differential gear ratio.” Also, after completion of the changeover operation of the first changeover mechanism, the rotational speed of the front-wheel-side propeller shaft becomes equal to the rotational speed of the rear-wheel-side propeller shaft However, even after completion of the changeover operation of the first changeover mechanism, if slippage in an acceleration direction is occurring at the left and right rear wheels, there may arise a situation where the rotational speed of the left and right rear wheels is greater than the rotational speed of the left and right front wheels, and the rotational speed of the front-wheel-side propeller shaft is greater than the “value obtained by multiplying the rotational speed of the left and right front wheels by the differential gear ratio.”
Because of the above-described phenomena, a situation where the rotational speeds of the first and second axles do not become (approximately) equal to each other may arise before and after completion of the changeover operation of the first changeover mechanism which is performed upon satisfaction of the above-described changeover condition. Accordingly, if a rotation synchronizing apparatus (synchronizer) is not provided, there may arise a situation where the connecting operation of the second changeover mechanism cannot be performed smoothly during traveling of the vehicle.